Of the human polyomaviruses, BK virus (BKV) and JC virus (JCV) were the first two identified. These two polyomavirus were isolated from immunosuppressed patients and published in the same issue of Lancet in 1971 (Gardner et al., Lancet 1971 1:1253-1527, and Padgett et al., Lancet 1971 1:1257-1260). Polyomaviruses are icosahedral, non-enveloped, double-stranded DNA viruses. They measure 40-45 nm in diameter and are comprised of 88% protein and 12% DNA.
The BKV genome is a circular double-stranded DNA of approximately 5 Kb in length and contains three major divisions: the early coding region, the late coding region, and a non-coding control region. The early coding region encodes for the three regulatory proteins (large tumor antigen [TAg], small tumor antigen [tAg], and truncated tumor antigen [truncTAg]), which are the first viral proteins expressed in a newly infected cell and are responsible for facilitating viral DNA replication and establishing a favorable cellular environment. The late coding region encodes the three structural proteins (VP1, VP2, and VP3) that make up the viral capsid, as well as the agnoprotein, the role of which during viral replication is less well-defined. The non-coding control region contains the origin of replication as well as the early and late promoters that drive expression of the viral gene products.
BKV has been detected in many different cell types including epithelial cells of the kidney, bladder, and ureter (typical sites of persistence), tonsillar tissue, and lymphocytes (proposed sites of primary infection and dissemination) (Chatterjee et al., J. Med. Virol. 2000; 60:353-362, Goudsmit et al., J. Med. Virol. 1982; 10:91-99, Heritage et al., J. Med. Virol. 1981; 8:143-150, Shinohara et al., J. Med. Virol. 1993; 41(4):301-305). The primary cell surface receptors for BKV are the gangliosides GT1b, GD1b, and GD3, all of which have a terminal α2,8-linked sialic acid and are fairly ubiquitous, allowing infection of various cell types (Neu et al., PLos Patholog. 2013; 9(10):e1003714 and e1003688, see also, O'Hara et al., Virus Res. 2014; 189:208-285). The non-enveloped icosahedral virion of BKV is composed of three different viral proteins: 360 copies of the major viral capsid protein VP1 arranged in 72 pentamers and 72 copies combined of the minor viral capsid proteins VP2 and VP3, with one VP2 or VP3 molecule associated with each VP1 pentamer. Only VP1 is exposed on the virion surface at entry and each pentamer has five low affinity binding sites for the ganglioside receptor. Binding of VP1 pentamers to ganglioside receptors on the cell surface initiates internalization through a caveolae-mediated endocytic pathway, followed by trafficking of the virus to the endoplasmic reticulum and finally to the nucleus (Tsai and Qian, J. Virol 2010; 84(19):9840-9852).
Infection with the human polyomavirus BK (BKV) is essentially ubiquitous, with estimates ranging between 80 and 90% of the population globally infected (Knowles W. A., Adv. Exp. Med. Biol. 2006; 577:19-45). Primary infection most often occurs during childhood (i.e., before age 10) and results in either a mild, non-specific, self-limited illness or no symptoms at all. Persistent infection is established in the epithelial cells of the renal tubules, ureters, and bladder, and is effectively controlled by the immune system. Transient asymptomatic viral shedding in the urine of immunocompetent adults occurs sporadically but results in no disease or sequelae. However, compromised immune function, particularly with immunosuppression following renal or hematopoietic stem cell transplantation, can lead to uncontrolled BKV replication and ultimately to BKV-associated nephropathy (BKVAN) or hemorrhagic cystitis (HC), a painful disease of the bladder. There are no effective antiviral therapies against BKV and the current standard of care is reduction of immunosuppression, which increases the risk of acute rejection. Even with the current, more aggressive approaches to monitoring and prevention, up to 10% of renal transplant recipients will develop BKVAN and 15-30% of those patients will suffer graft loss due to BKVAN. Among those undergoing reduction in immunosuppressive regimen upon detection of BKV viremia, up to 30% will experience an acute rejection episode as a result.
Although BKV was first described in 1971 (supra), it was not until the 1990s that BK associated nephropathy (BKVAN) was reported in the literature as a cause of kidney transplant injury (Purighalla et al., Am. J. Kidney Dis. 1995; 26:671-673 and Randhawa et al., Transplantation 1999; 67:103-109). In early management of BKVAN, testing positive for BK had severe consequences, with more than 50% of the patients having graft dysfunction and graft loss (Hirsch et al., New Engl. J. Med. 2002; 347:488-496). BK viral reactivation may begin after transplantation, and is seen in about 30%-50% of the patients by 3 months post-transplantation (Bressollette-Bodin et al., Am J. Transplant. 2005; 5(8):1926-1933 and Brennan et al., Am. J. Transplant. 2004; 4(12):2132-2134). BK viral reactivation can be first seen by virus and viral DNA in the urine, then in the plasma and finally in the kidney. (Brennan et al., Am. J. Transplant. 2005; 5(3):582-594 and Hirsch et al., N Eng. J. Med. 2002; 347(7):488-496). About 80% of kidney transplant patients have BK virus in the urine (BK viruria) and 5-10% of these patients progress to BKVAN (Binet et al., Transplantation 1999; 67(6):918-922 and Bressollette-Bodin et al., Am J. Transplant. 2005; 5(8):1926-1933). BKV effects the renal tubular epithelial cells causing necrosis and lytic destruction with denudation of the basement membrane, which allows tubular fluid to accumulate in the interstitum, which results in interstitial fibrosis and tubular atrophy (Nickeleit et al., J. Am. Soc. Neprol. 1999; 10(5):1080-1089) all of which can affect the condition of the transplant. Patients may present with deterioration of renal function, tubule-interstitial nephritis and ureteric stenosis (Garner et al., Lancet 1971; 1(7712):1253-1257 and Hirsch Am. J. Transplant 2002; 2(1)25-30).
BKV can also cause pneumonitis, retinitis and meningoencephalitis in immunocompromised hosts (Reploeg et al., Clin. Infect. Dis. 2001; 33(2):191-202). BKV disease in hematopoietic stem cell transplant (HSCT) recipients typically manifests as hemorrhagic cystitis (HC), which can vary in severity. Viruria (but not viremia) and painful hematuria are associated with the clinical presentation of HC. The current standard of care is supportive in nature, involving primarily forced hydration/diuresis and pain management measures. The most severe cases require blood transfusions, clot evacuation, and can lead to death in some instances. HC of any cause (e.g. drug, radiation, viral) is relatively common among HSCT recipients but BKV-associated HC occurs in approximately 10-12% of patients usually within 6 months after transplantation. There are other viral etiologies of HC, with adenovirus being a more common cause of HC among pediatric HSCT recipients compared with adult HSCT recipients. BK virus has also been observed in other immunocompromised conditions such as systemic lupus erythromatosis, other solid organ transplants and in HIV/AIDS patients (Jiang et al., Virol. 2009; 384:266-273).
At this point, the treatment of BK nephropathy associated with organ transplantation is the reduction of immunosuppression in an attempt to prevent graft dysfunction and graft loss (Wiseman et al., Am. J. Kidney Dis. 2009; 54(1): 131-142 and Hirsch et al., Transplantation 2005; 79(1): 1277-1286). There are no fixed clinical regimes for the reduction, as reduction of the immunosuppression may help to prevent progression from viremia to the extensive damage associated with clinical nephropathy, but this also increases the risk of acute organ rejection (Brennan et al., Am. J. Transplant 2005; 5(3):582-594). Clinicians have reported the use of therapeutics such as cidofovir, leflunomide or quinolones in combination with the reduction of immunosuppressants, however the reports find this approach ineffective, with the added burden of managing additional side effects (Randhawa and Brennan Am. J. Transplant 2006; 6(9):2000-2005). As such, there is an unmet and useful need in the field for therapies that neutralize polyoma viruses such as BK and that can be used in an immunocompromised host.
JC virus is also a polyoma virus which is also highly prevalent in the population (80%), although JC virus is generally acquired later than BK virus (Padgett et al., J. Infect. Dis. 1973; 127(4):467-470 and Sabath et al., J. Infect. Dis. 2002; 186 Suppl. 2:5180-5186). After initial infection, JC virus establishes latency in the lymphoid organs and kidneys and when reactivated, invades the central nervous system via infected B-lymphocytes. Once in the CNS, the JC virus causes progressive multifocal leukoencephalopathy (PML), which is a progressive demylenating central nervous system disorder. PML most often presents as an opportunistic infection in HIV/AIDS patients and has also been reported in immunosuppressed patients (Angstrom et al., Brain 1958; 81(1):93-111 and Garcia-Suarez et al., Am. J. Hematol. 2005; 80(4):271-281). PML patients present with confusion, mental status changes, gait ataxia, focal neurological defects such as hemi paresis, limb paresis and visual changes (Richardson E. P., N. Eng. J. Med. 1961; 265:815-823). The prognosis of patients with PML is poor and is especially poor in patients with HIV/AIDS (Antinori et al., J. Neurovirol. 2003; 9 suppl. 1:47-53). This further highlights the unmet and useful need in the field for therapies that neutralize polyoma viruses such as JC.